1. Field of the Invention
The present invention relates to a differential action control system and, in particular, to a vehicle including respective differential mechanisms between both front and rear wheels and between a front axle and a rear axle.
2. Description of the Related Art
A so called four wheel drive vehicle is provided with a center differential mechanism disposed between a front drive axle and a rear drive axle as well as a front differential mechanism disposed between front wheels and a rear differential mechanism disposed between rear wheels. It is possible for the four wheel vehicle to provide a proper differential action to not only the front wheels and the rear wheels but also between the front axle and rear axle so as to eliminate so called a tight corner braking action by utilizing these differential mechanisms.
It should, however, be noted that it is difficult for the conventional four wheel vehicle to make a stable starting action or stable acceleration if one of the wheels of the vehicle starts racing. This is because the engine drive torque is hard to properly transmit to wheels other than the racing wheel. As a result, the driving stability, braking and accelerating characteristics are deteriorated.
In view of this, Japanese patent Public Disclosure No. 62-166114, laid open to the public in 1987, discloses a differential action control device for locking and unlocking the front, rear and/or center differential mechanisms in accordance with a driving condition of the vehicle by virtue of a hydraulic pressure control.
The differential action control device receives signals of wheel speed of the respective wheels and steering angle in the control circuit, judges whether the vehicle is running on a rough road, or making a straight running, accelerating action or braking action based on the signal and controls the front, rear and center differentials so as to improve the steering stability, braking characteristics and acceleration characteristics.
It should be noted that the conventional differential action control devices tend to make a locking of two or more differentials concurrently when a differential is to be locked. Thus, the amount of the torque transmitting to the wheels is abruptly changed to cause a so called torque shock. In the differential action control device as disclosed in the above Japanese Patent Public disclosure No. 62-166114, the differential action control is made in view of a specific running condition of the vehicle, such as acceleration, deceleration and the like. Consequently, this does not necessarily accomplish a smooth differential action control as a whole to satisfy the driver's feeling.
In addition, in the differential action control device as disclosed in the Japanese Patent Public Disclosure 62-166114, each of the differentials is controlled in accordance with various control parameters. However, it is desirable to detect the control conditions of the differentials with each other and control the respective differentials in view of an entire vehicle drivability. If each differential is controlled independently from the other, the active drivability, steering characteristics and driving stability may be deteriorated.
For instance, in a four wheel drive vehicle in which the driving force distribution ratio for the front and rear differentials is equal when the center differential is locked, and the driving force distribution for the front differential is reduced relative to that for the rear differential when the center differential is made free, an oversteer trend is enhanced if the driving force distribution is increased for the rear differential when a slip condition has occurred in the one of the rear wheels when the center differential is unlocked. The reason for that is as follows. Where the torque distribution ratio for the wheels are changed, the driving force transmitted from the each of the wheels to the road surface is also changed even though the engine output is not changed. It should, however, be understood that the capacity of the wheel, that is, the total amount of the driving force which can be transmitted from each of the wheels to the road surface, is substantially constant whether or not the direction of the force introduced into the wheel is varied (this phenomenon is commonly explained by using a friction circle). Consequently, if the distribution of the driving force for the rear wheels is increased, the grip force or resistance force of the rear wheels against a side force acting on the vehicle is reduced so as to tend to make the vehicle oversteer abruptly. It is desirable that the differential action control conditions for the respective differentials be detected or monitored to account for making a total control for the differential action of the vehicle.